Add Gamepad button mapping support

[nkymut]

• Added support for different gamepad button mappings (XBOX, NES, custom).
• Refactored global buttonMap to instance variable to support multiple gamepad mappings

Usage

const pad1 = gamepad(0); // Default mapping is XBOX {a: 0, b: 1, x: 2, y: 3}
const pad2 = gamepad(1, 'XBOX'); // XBOX button mapping {a: 0, b: 1, x: 2, y: 3}
const pad3 = gamepad(2, 'NES'); // Nintendo button mapping {a: 1, b: 0, x: 3, y: 2}
const pad4 = gamepad(3, {a: 0, b: 1, x: 2, y: 3}); // Custom mapping